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 Sclerotia February 2000 After a devastating year of white mold head rot in sunflower in the Northern Plains, many producers are asking the obvious question: "What do I do with that field which now has high populations of sclerotia in the soil?" Affected producers know that at a minimum, these small, dark and hard sclerotia bodies can threaten broadleaf crops for the next several years. But is there a tillage or cropping system that might be helpful in accelerating the reduction of the sclerotia populations that now reside in the soil? The answer is a qualified "yes." But plant pathologists are quick to add that this disease seems to possess the proverbial "nine lives." One reasonable option might be to leave the sclerotia on the soil surface and plant the infested ground to a small grain under a minimum-tillage program. According to Craig Grau, plant pathologist at the University of Wisconsin, the goal is to reduce that disease load as soon as possible. "Reducing the number of sclerotia -just like depleting a weed seed bank - contributes to the control of white mold," Grau explains. The mantra of no-tillers calls for leaving weed seeds on the soil surface. Burying the weed seeds will ensure that unwanted plants will be popping out of the soil surface when you can least afford them. Burying sclerotia may result in a similar situation: a problem to be faced years later. The heavy incidence of Sclerotinia head rot in 1999 put literally tons of sclerotia into the soils of the Dakotas and Minnesota. Sclerotinia sclerotia attack most broadleaf crops (and weeds) upon the formation of mushroom-like structures called "apothecia." Conditions conducive to their formation would include adequate moisture, a shady environment and sufficiently warm temperatures. The apothecia, in turn, produce numerous spores that are ejected into the air and disseminated to a host plant. Among the highly susceptible crops are sunflower, soybeans, edible beans, canola, peanuts, and many broadleaf vegetable crops. (Corn, small grains and forage grasses are on the list of non-host crops.) Susceptible weeds include lambsquarter, redroot pigweed, common ragweed, velvetleaf and wild mustard, among others. According to North Dakota State University's Sunflower Production (Extension Bulletin #25/Revised), the spores attack dead tissue or senescing flower petals and florets of the susceptible plants. Infection also may occur on damaged leaves or in wounds of the host plants. Should wet conditions persist, the disease grows on the host plant using the dead tissue as a food base to initiate growth - and the cycle continues to evolve with development of the sclerotia bodies that fall to the soil. Once on or in the soil, the sclerotia are subject to natural antagonists. If left on or near the soil surface, the sclerotia are subject to the Northern Plains' harsh weathering of freezing, drying and wetting. Cracked or broken sclerotia may be especially vulnerable to soil bacteria and other soil-borne antagonists. The sclerotia must go through a "conditioning process" that makes them vulnerable to antagonists and the eventual apothecia formation, according to Berlin Nelson, NDSU plant pathologist. Nelson says four factors are required for sclerotia to form apothecia: "You need wet soil conditions for at least seven days, 'preconditioned' sclerotia, a shaded environment at the soil surface, and soil temperatures warm enough for seed germination." Nelson and Grau point out that when it comes to management, sclerotia must be viewed in terms of their total population, not as individual bodies. Reducing that general population as quickly as possible reduces the risk of the disease in the future. In addressing the viability of tillage as a control measure, Wisconsin's Grau refers to experiences in his state's soybeans. "We had a huge white mold problem in 1994 in Wisconsin soybeans. The following year we set up three tillage systems," he explains. The first approach was a deep plow (eight-inch depth); the second, a chisel plow; and the third was a no-till system. Sclerotia densities under each system were observed over a three-year period at three locations. The numbers below summarize the Wisconsin study: NUMBER OF SCLEROTIA PER LITER OF SOIL 1995 1996 1997 Tillage System No Till 4.3 1.5 0.6 Chisel 4.1 1.8 1.0 Moldboard 5.1 2.2 1.2 Why the Difference? Under a no-till system, allowing the sclerotia to remain on the soil surface exposes them to weathering and decay. In comparison, with deep plowing the sclerotia are protected from the natural soil-surface sequence of weathering. Equally as important, according to Grau, is that sclerotia on or near the soil surface are vulnerable to germination (i.e., production of apothecia) during the next cropping season. Once the sclerotia have produced apothecia, they've usually used up their internal food source. So the critical step is to create an environment conducive to germinating the sclerotia when other broadleaf crops are not present or at a stage vulnerable to infection. Grau and Nelson agree that the best crop to plant following a year of heavy sclerotia production is a small grain such as wheat, barley or oats. At normal or higher populations and growth, these are heavy-canopy crops. (Remember: sclerotia need wet soils and a shady environment to germinate and form apothecia.) Of the various non-host crops, the heavy small grain canopy has the best potential to create the microclimate required for germination. In his work, for instance, Grau found that an early April planting of oats developed an acceptable canopy by late May and early June. The heavy grain canopy keeps air movement at the field surface to a minimum. Late May and early June offer a high probability of rainfall in the Northern Plains, and mornings are generally accompanied by heavy dew. The small grain canopy present by that time provides the necessary shading required for sclerotia germination. Should surface-level soils remain wet for up to seven days and the sclerotia are on or near the soil surface, the likelihood of sclerotia germination is high. Also, Grau points out, the spores produced by the germinating sclerotia are unlikely to find a suitable host as of May or June. And without a host, a spore's life is quite short. (So broadleaf weed control is critical at this point.) Most of the susceptible crops are vulnerable in July and August. With sunflower head rot disease, the vulnerable period is during the latter stage of pollination when the petals and florets begin to dry, according to Tom Gulya, Fargo-based USDA-ARS plant pathologist. That's usually in mid- to late August. NDSU's Nelson points out that once the small grain field is harvested, the required shading factor has been eliminated and sclerotia are not as likely to germinate - even if wet conditions exist in late August. Spores can travel a long distance with the wind, so neighboring fields can be infected. An already-harvested small grain crop will minimize spore production in that field even if conditions are good for apothecia production, Grau notes. Corn is not as good a choice for a succeeding crop on a field with heavy sclerotia populations. "By the time the corn field has a heavy canopy in middle July, most neighboring susceptible crops would be vulnerable to infection," according to Grau. Sclerotia aren't all created equal. "Not all of the sclerotia will germinate on the soil surface, even if the conditions are right," Nelson says. That reality leaves a population for succeeding years. But, keeping sclerotia on the soil surface keeps them vulnerable to degradation by weathering and other natural antagonists. Deep plowing will, of course, bury sclerotia situated at or near the soil surface. However, the Wisconsin research found that later tillage, in turn, brought the sclerotia back to the soil surface. Grau stresses if plowing is used to bury the sclerotia, it is important not to deep plow in succeeding years since it is necessary to keep the sclerotia buried. Complicating that strategy, though, is the fact that buried sclerotia can remain viable for many years, thus threatening sunflower planted in succeeding rotations. Along with the head rot form, sunflower also is vulnerable to Sclerotinia wilt, a disease form that results when sunflower roots come into contact with sclerotia in the soil. Those sclerotia germinate and then infect and decay the sunflower root. The fungus moves into the stem, and the plant wilts and dies. Sunflower is the only crop that is infected by this form of the disease - which is why deep plowing as a method to prevent aeciospore development on the soil surface can haunt the sunflower producer in later rotations via root contact. Success at reducing the impact of any plant pest comes by attacking its weakest, most-vulnerable stage. In the case of sclerotia, it appears vulnerability is greatest when they are left on or near the soil surface. But researchers hasten to point out that Sclerotinia is a very vexing disease to manage. With so many crops impacted by Sclerotinia white mold, the combination of more acres of susceptible crops and tighter rotations between them becomes a big part of the problem. Wisconsin researchers believe that narrow row spacings, coupled with higher soil fertility, have added to the problem of Sclerotinia in soybeans. Also, current higher-yielding varieties of soybeans may have less resistance. (In general, management practices which increase soybean yield potential all result in faster and more-complete canopy closure, Craig Grau points out - and canopy density is what really drives the pathogen and the disease it triggers.) The issue of Sclerotinia white mold disease is a national issue, not just one impacting the sunflower industry. Various commodity groups are talking to each other about the possibility of a national initiative to deal with this problem. For affected growers, that can't happen too soon. - Larry Kleingartner  Back to Disease Stories Back to Archive Categories
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